An internal combustion engine generates power by burning a fuel in a combustion chamber by drawing air into the chamber. Intake valves intake the air by rotation of a camshaft, and the air is drawn into the combustion chamber while the intake valves are opened. In addition, exhaust valves allows combustion gas out by the rotation of the camshaft, and the gas is exhausted from the combustion chamber while the exhaust valves are opened.
The optimal operation of the intake valves and the exhaust valves depends on a rotation speed of the engine. In order to achieve such an optimal valve operation depending on the rotation speed of the engine, various researches, such as designing of a plurality of cams and a continuous variable valve lift (CVVL) that can change valve lift according to engine speed, have been undertaken.
Further, in order to achieve such an optimal valve operation depending on the rotation speed of the engine, a continuously variable valve timing (CVVT) apparatus that enables different valve timing operations depending on the engine speed has been developed. The general CVVT may change valve timing with a fixed valve opening duration.
However, the general CVVL and CVVT have a complicated structure and manufacturing cost is high.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.